Intensity-modulating optical fiber sensors are put to many practical uses when the advantages inherent in optical detection and transmission (such as, for example, immunity to electromagnetic effects, safety factor in an explosive or inflammable atmosphere, miniaturization, simplicity and low cost, etc.) are needed. However, the use of this type of sensor is limited by virtue of its low degree of linearity, its lack of accuracy and above all the fact that the light intensity which constitutes the information from the sensor may be disturbed all along the path between the light source, the sensor and the detector. Moreover, the signal received, for a given sensor, depends on the length of the transmission line between the emitter and the sensor. D.E.N. Davies et al, in an article entitled `Displacement Sensor Using a Compensated Fibre Link,` which was presented at the second international conference relating to optical fiber sensors, held in Stuttgart, in Sept. 1984, have proposed a system incorporating a displacement sensor and in which means are provided to compensate for the effects of a variation in the power of the light source, the sensitivity of the receiver or attenuation of the fiber. This compensation effect is achieved by the incident light being divided between two paths, one of which incorporates the sensor itself while the other is used as a reference path. The system described requires a complicated sensing cell comprising a coupler, two fibers and a set of lenses. Such a construction is expensive and is therefore poorly suited to uses in which a large number of sensors would be required.